The hydrometallurgical recovery of metals often comprises the following stages: the leaching of a concentrate or ore, liquid-liquid extraction and the precipitation or reduction of the metal. Leaching may be bioleaching or dilute acid leaching, from which the aqueous solution is routed to liquid-liquid extraction. In liquid-liquid extraction, an organic reagent solution is mixed in an extraction cell (mixer-settler) or in a column, into an aqueous solution that contains the substance to be purified and concentrated as soluble, metal usually in the ion form or as a complex along with several impurities. The valuable metal or substance to be refined reacts selectively with the organic extraction chemical, whereby it is made to separate from the aqueous solution into the extraction chemical in pure form. The valuable metal or substance can then be separated from the organic solution back into an aqueous solution (stripping) with the inverted chemical reaction to extraction, and can then be recovered again from there as a product, for instance by precipitation or reduction to metal. Reduction is usually electrolysis, to which the stripping aqueous solution is routed as the electrolyte. When the valuable metal is recovered from the electrolyte, this acidic aqueous solution is routed back to stripping.
The extraction process is thus the mixing together of liquids that are physically insoluble in each other into droplets or a dispersion in the mixing section of the extraction apparatus, and after chemical mass transfer, the droplets in the dispersion are made to coalesce back into the original layers of liquid in the settling section or settler. Intensive mixing or a significant change in the surface chemistry conditions of the process may result in very small droplets, which require a lot of time to disengage to their own liquid phase. These droplets do not necessarily have time to disengage in the actual settling section of the extraction step, but move further along the process with the other phase. The entrainment of the original feed solution (aqueous solution) with the organic solution to later process steps may reduce the purity of the final product and require additional purification measures. This of course increases process costs. In the same way organic extractant may get lost together with the treated aqueous solution. In both cases the cost-effectiveness of the process is reduced.
In the copper extraction process, impurities may be transferred via stripping to the copper electrolyte, endangering the quality of the copper cathodes in the subsequent electrolysis. It is also quite expensive to discard part of the sulphuric acid-containing electrolyte continually, which is for the time being the general practice for reducing impurity levels.
In particular, a tank has been used for the removal of aqueous entrainment from the organic solution situated after the extraction cells, in which the residual droplets of water fall towards the bottom of the tank by the effect of gravity and the purified surface layer of the solution can be routed to the following process stage, known as a after-settler. The tank may function simultaneously as a equalising tank, which is required to even out changes in the volume of organic solution that occur in the various parts of the process. In this case the surface level of the solution in the tank varies.
The actual purification step of the organic solution, scrubbing, takes place using mixer-settler cells, in which basically chemically-bound impure substances are removed by treating the organic solution with a suitable aqueous solution. In this case therefore a dispersion of extraction solution and aqueous solutions, similar in function to an extraction cell, is formed so as to obtain a large liquid-liquid surface area. Besides chemical scrubbing, water droplets are also removed or the impurities contained in them are diluted. A mixer-settler cell built for scrubbing purposes generally consists of a pump, a mixer and a settling tank with its retaining fences and is generally the size of an extraction cell. Changes in organic solution volume cannot be evened out with a scrubbing cell, so a separate equalising tank as mentioned above is needed, with the required volume capacity.
The scrubbing of the extraction solution described above has been adopted in extraction solutions in order to dilute the water droplets that are entrained in them. In general, the scrubbing solution contains sulphuric acid and, in the case of copper extraction, copper too, so that impurities such as iron bound to the extraction solution via chemical reactions may be displaced by this means. Even after the mixer-settler of the scrubbing stage the extraction solution may still contain some residual droplets of water, and their removal also prevents to some extent the transfer of impurities to the following process stage.